Electrical Testing

For any electrical system to work, electricity must be able to flow in a complete circuit from the power source (the battery) via the components and back to the battery. No circuit means no electrical flow. Once the power has left the positive side of the battery and run through the component it must then return to the battery on its negative side (this is called earth or ground). To save on wiring, connections and space, the negative side of the battery is connected directly to the frame or engine. Around the frame and engine will be various other ground points to which the wiring coming from components will be connected. In the case of the starter motor it bolts directly to the engine, which is bolted to the frame. Therefore the frame and engine also form part of the earth return path.

Ohm’s Law

The relationship between voltage, current and resistance is defined by Ohm’s Law.

  • The potential of a battery is measured in volts (V).

  • The flow of current in a circuit (I) is measured in amperes.

  • The power rating of a consumer is measured in watts (W).

  • The resistance (R) of a circuit is measured in Ohms.

Ohm’s law, for practical work can be described as -

Ohms Law 1 Electrical Testing

Power is calculated by multiplying Volts x Amps -

Ohms Law 2 electrical Testing

By transposing either of these formulae, the value of any unit can be calculated if the other two values are known.

For example, if a battery of 12 V is connected to a bulb of 60 W:

  • the current flowing in the circuit can be calculated by using -

    • Ohms Law 3 Electricat Testing

  • the bulb resistance can be calculated by using -

Ohms law 4_1 Electrical Testing

To use either of the following triangles, put your finger over the value you want to find. Multiply the remaining values if side by side, or divide if one is over the other.

Ohms Law 5_1 Electrical Testing

Basic Electrical Circuits

Basic Circuit Electrical Testing

Basic Circuit Diagram

In the above circuit an electrical reservoir (the battery) is connected via a cable to a terminal on the controlling device (the switch) whose contacts are either open or closed. The other terminal on the switch is connected via a cable to the consumer (the bulb), and the other side of the bulb filament is connected to ground (earth) by another cable. The ground point is usually a part of the frame or engine, to which the battery negative terminal is also connected.

When the switch contacts are open (as shown in the diagram), the circuit is broken and no current flows. When the switch contacts are closed, the circuit is made and current flows from the battery positive terminal through the switch contacts and bulb filament to ground. The frame completes the circuit to the battery negative terminal and the bulb illuminates.

Although some circuits on the circuit diagram may at first seem more complicated, it will generally be found that they can be broken down into sections which do not differ greatly from the basic circuit above.